Non-coding Intergenic Spacer Research Articles

Comparative Mitogenome Analyses Uncover Mitogenome Features and Phylogenetic Implications of the Parrotfishes (Perciformes: Scaridae).

In order to investigate the molecular evolution of mitogenomes among the family Scaridae, the complete mitogenome sequences of twelve parrotfish species were determined and compared with those of seven other parrotfish species. The comparative analysis revealed that the general features and organization of the mitogenome were similar among the 19 parrotfish species. The base composition was similar among the parrotfishes, with the exception of the genus Calotomus, which exhibited an unusual negative AT skew in the whole mitogenome. The PCGs showed similar codon usage, and all of them underwent a strong purifying selection. The gene rearrangement typical of the parrotfishes was detected, with the tRNAMet inserted between the tRNAIle and tRNAGln, and the tRNAGln was followed by a putative tRNAMet pseudogene. The parrotfish mitogenomes displayed conserved gene overlaps and secondary structure in most tRNA genes, while the non-coding intergenic spacers varied among species. Phylogenetic analysis based on the thirteen PCGs and two rRNAs strongly supported the hypothesis that the parrotfishes could be subdivided into two clades with distinct ecological adaptations. The early divergence of the sea grass and coral reef clades occurred in the late Oligocene, probably related to the expansion of sea grass habitat. Later diversification within the coral reef clade could be dated back to the Miocene, likely associated with the geomorphology alternation since the closing of the Tethys Ocean. This work provided fundamental molecular data that will be useful for species identification, conservation, and further studies on the evolution of parrotfishes.

FIRST FINDING OF <i>SOLIDAGO </i><i>×</i><i> NIEDEREDERI</i> IN TULA OBLAST (EUROPEAN PART OF RUSSIA)

Invasive plant species Solidago canadensis , widespread in Russia, occasionally forms hybrids S. × niederederi with the native species S. virgaurea . Previous findings of this hybrid were known mainly for the North-Western regions of Russia. Plants, morphologically similar to S. × niederederi, were found in August 2020 in the city of Aleksin in the Tula region. The hybrid origin of these plants was proved by the analysis of the nuclear ribosomal internal transcribed spacer 1-2 (ITS1-2). Analysis of the chloroplast non-coding intergenic spacer rpl 32- trn L showed that both parent species could act as both maternal and paternal ones in the same population. The available data suggest that individuals of S. × niederederi arise every time as a result of new hybridization events.

The mitochondrial genome of the semi-slug Omalonyx unguis (Gastropoda: Succineidae) and the phylogenetic relationships within Stylommatophora.

Here we report the first complete mitochondrial genome of the semi-slug Omalonyx unguis (d'Orbigny, 1836) (Gastropoda: Succineidae). Sequencing was performed on a specimen from Argentina. Assembly was performed using Sanger data and Illumina next generation sequencing (NGS). The mitogenome was 13,984 bp in length and encoded the 37 typical Metazoan genes. A potential origin for mitochondrial DNA replication was found in a non-coding intergenic spacer (49 bp) located between cox3 and tRNA-Ile genes, and its secondary structure was characterized. Secondary structure models of the tRNA genes of O. unguis largely agreed with those proposed for other mollusks. Secondary structure models for the two rRNA genes were also obtained. To our knowledge, the 12S-rRNA model derived here is the first complete one available for mollusks. Phylogenetic analyses based on the mitogenomes of O. unguis and 37 other species of Stylommatophora were performed using amino acid sequences from the 13 protein-coding genes. Our results located Succineoidea as a sister group of Helicoidea + Urocoptoidea, similar to previous studies based on mitochondrial genomes. The gene arrangement of O. unguis was identical to that reported for another species of Succineoidea. The unique rearrangements observed for this group within Stylommatophora, may constitute synapomorphies for the superfamily.

Characterization of complete mitochondrial genome of Polyura narcaeus (Lepidoptera: Nymphalidae: Charaxinae)

The complete mitochondrial genome of Polyura narcaeus (Hewitson, 1854) (Lepidoptera: Nymphalidae: Charaxinae) was sequenced in the study. The circular genome is 15,319 bp in size and includes 13 protein-coding genes, 22 transfer RNA genes, 2 ribosomal RNA genes and a non-coding AT-rich region. The base composition of the whole mitogenome is 39.15% A, 42.08% T, 11.18% C and 7.59% G, showing a strong AT bias. The characteristics of encoding PCGs, rRNAs and tRNAs, as well as the non-coding intergenic spacers and overlapping sequences are nearly the same with other known butterflies. The AT-rich region also contains several features characteristic of the typical butterflies. Phylogenetic analysis distinctly showed that the family Nymphalidae was a monophyletic group, and that the newly determined Polyura narcaeus of this study was firstly sister to Polyura nepenthes, then they were clustered with Polyura arja.

Chloroplast Genomes of the Green-Tide Forming Alga Ulva compressa: Comparative Chloroplast Genomics in the Genus Ulva (Ulvophyceae, Chlorophyta)

To understand the evolution of Ulva chloroplast genomes at intraspecific and interspecific levels, in this study, three complete chloroplast genomes of Ulva compressa Linnaeus were sequenced and compared with the available Ulva cpDNA data. Our comparative analyses unveiled many noticeable findings. First, genome size variations of Ulva cpDNAs at intraspecific and interspecific levels were mainly caused by differences in gain or loss of group I/II introns, integration of foreign DNA fragments, and content of non-coding intergenic spacer regions. Second, chloroplast genomes of U. compressa shared the same 100 conserved genes as other Ulva cpDNA, whereas Ulva flexuosa appears to be the only Ulva species with the minD gene retained in its cpDNA. Third, five types of group I introns, most of which carry a LAGLIDADG or GIY-YIG homing endonuclease, and three of group II introns, usually encoding a reverse transcriptase/maturase, were detected at 26 insertion sites of 14 host genes in the 23 Ulva chloroplast genomes, and many intron insertion-sites have been found for the first time in Chlorophyta. Fourth, one degenerate group II intron previously ignored has been detected in the infA genes of all Ulva species, but not in the closest neighbor, Pseudoneochloris marina, and the other chlorophycean taxa, indicating that it should be the result of an independent invasion event that occurred in a common ancestor of Ulva species. Finally, the seven U. compressa cpDNAs represented a novel gene order which was different from that of other Ulva cpDNAs. The structure of Ulva chloroplast genomes is not conserved, but remarkably plastic, due to multiple rearrangement events.

New data on the mitochondrial genome of the winter crane fly (Diptera: Trichoceridae) with phylogenetic analysis

The family Trichoceridae is a small group in Nematocera (Diptera) with 157 known species in the world. In this study, we report a complete mitochondrial genome sequence of a Trichoceridae species, which is a circular molecule of 16,094 bp with 13 protein-coding genes (PCGs), 22 transfer RNA genes (tRNAs), two ribosomal RNA genes (rRNAs), and a 1063 bp long non-coding region (control region). Eight gene overlaps ranging from 1 to 8 bp in length and 16 small non-coding intergenic spacers ranging from 1 to 77 bp in length are found in the mitochondrial genome. The canonical start codons (ATN) for invertebrate mitochondrial genomes are found in 11 PCGs, except for COI which uses TTG and ND5 uses GTG. Stop codons of 12 PCGs are invariably complete TAA and TAG, while COII ends with a single thymine stop codon. Twenty-two tRNAs ranges from 64 bp to 72 bp, and the rRNAs are determined to be 1326 bp in length for lrRNA and 788 bp in length for srRNA. Phylogenetic analysis reveals that the family Trichoceridae is a sister-group to the remaining Tipulomorpha, and the relationship between the other families is Pediciidae + (Limoniidae + (Cylindrotomidae + Tipulidae)).

TBX1 Functions as a Tumor Activator in Prostate Cancer by Promoting Ribosome RNA Gene Transcription.

TBX1 belongs to an evolutionarily conserved family of transcription factors involved in organ development. TBX1 has been reported to have a hypermethylated cytosine guanine dinucleotide island around its second exon, which was related to prostate cancer (PCa) progression. However, the role and exact mechanism of TBX1 in PCa remains unknown. Using human prostate samples, online data mining and multiple in vitro and in vivo models, we examined the biological role and underlying mechanisms of TBX1 in PCa. TBX1 was highly expressed in PCa tissues, and high TBX1 expression was positively associated with Gleason score, pathological tumor stage, pathological lymph node stage, extraprostatic extension and disease/progression-free survival. In vitro and in vivo data demonstrated that TBX1 silencing inhibits PCa cell proliferation and colony formation and increases the cell population at the G0/G1 phase. The exogenous expression of TBX1 rescued these phenotypes. Mechanistically, TBX1 silencing suppressed the expression of 45S ribosomal RNA (rRNA), which was rescued by the exogenous expression of TBX1. TBX1 silencing inhibited the monomethylation of histone 3 lysine 4 (H3K4me1) binding with the non-coding intergenic spacer (IGS) regions of ribosomal DNA (rDNA) and the recruitment of upstream binding factor to the promoter and IGS regions of rDNA. The drug-induced enhancement of H3K4me1 counteracted the effect of TBX1 silencing. These findings indicate that TBX1 exerts its tumor activator function in PCa cells via epigenetic control, thereby promoting rRNA gene transcription. Thus, TBX1 may represent a prognostic biomarker and therapeutic target for PCa patients.

Comparative mitogenome analyses uncover mitogenome features and phylogenetic implications of the subfamily Cobitinae

BackgroundLoaches of Cobitinae, widely distributed in Eurasian continent, have high economic, ornamental and scientific value. However, the phylogeny of Cobitinae fishes within genera or family level remains complex and controversial. Up to now, about 60 Cobitinae mitogenomes had been deposited in GenBank, but their integrated characteristics were not elaborated.ResultsIn this study, we sequenced and analyzed the complete mitogenomes of a female Cobits macrostigma. Then we conducted a comparative mitogenome analysis and revealed the conserved and unique characteristics of 58 Cobitinae mitogenomes, including C. macrostigma. Cobitinae mitogenomes display highly conserved tRNA secondary structure, overlaps and non-coding intergenic spacers. In addition, distinct base compositions were observed among different genus and significantly negative linear correlation between AT% and AT-skew were found among Cobitinae, genus Cobitis and Pangio mitogenomes, respectively. A specific 3 bp insertion (GCA) in the atp8-atp6 overlap was identified as a unique feature of loaches, compared to other Cypriniformes fish. Additionally, all protein coding genes underwent a strong purifying selection. Phylogenetic analysis strongly supported the paraphyly of Cobitis and polyphyly of Misgurnus. The strict molecular clock predicted that Cobitinae might have split into northern and southern lineages in the late Eocene (42.11 Ma), furthermore, mtDNA introgression might occur (14.40 Ma) between ancestral species of Cobitis and ancestral species of Misgurnus.ConclusionsThe current study represents the first comparative mitogenomic and phylogenetic analyses within Cobitinae and provides new insights into the mitogenome features and evolution of fishes belonging to the cobitinae family.

Comparative analysis of the constituents of two cultivars of Dioscorea dumetorum (Kunth) Pax. and their molecular barcoding

The edible and wild cultivars of Dioscorea dumetorum (Kunth) Pax. (Family: Dioscoreaceae) are commonly used as close substitutes in herbal markets due to their edibility and medicinal importance. Comparative study of the cultivars was done using chemical analysis of their methanol tuber extracts and by utilizing the DNA barcode regions, ribulose-1, 5-bisphosphate carboxylase subunits (rbcL) and the non-coding intergenic spacer gene (trnH-psbA) for nucleotide sequences comparison of cultivars. Of the 54 compounds identified from the methanol extract of the edible and wild cultivars using gas chromatography-mass spectrometry (GC-MS), 12 are similar, while 42 are remarkably different. In particular n-hexadecanoic (palmitic) acid had a higher percentage area in the edible cultivar (31.16%) compared to the wild cultivar (0.26%). The cultivars were successfully amplified using the universal primers gene rbcL and trnH-psbA. Their nucleotide sequences showed a slight variation when aligned with CodonCode Aligner V.9.0.1. They were identified by the Basic Local Alignment Search Tool (BLAST) through comparison with Genbank data, which shows close similarities with submitted data. The use of molecular barcoding in identifying the cultivars of Dioscorea dumetorum as well as the chemical composition of their tubers may form an important marker in the compilation of future herbal pharmacopoeia for its proper identification.

Generation of novelties in the genus Ocimum as a result of natural hybridization: A morphological, genetical and chemical appraisal

The genus Ocimum is a boutique of a diverse set of specialized metabolites such as terpenoids and phenylpropanoids. Each Ocimum species and its cultivars represent a characteristic chemical profile. The present study explored the two interspecific Ocimum hybrids originating through a serendipitous natural cross between O. kilimandscharicum and O. basilicum. These two novel Ocimum hybrids exhibited intermediate morphological features of two parental species. Inter simple sequence repeats (ISSR) analysis and DNA barcoding with the plastid non-coding trnH-psbA intergenic spacer region reaffirmed unambiguous parental identification and differentiation of these natural hybrids from other available Ocimum species. Consequently, gas chromatography-mass spectrometry-based metabolite profiling of two hybrids identified them as specific chemotypes with the presence of a unique blend of specialized metabolites from the parental species, which are either rich in terpenes or phenylpropanoids. Additionally, expression analysis of key genes from terpenoid and phenylpropanoid pathways corroborated with differential metabolite accumulation. Thus, these two Ocimum hybrids represented the novel chemotypes, which could be useful in commercial cultivation to produce novel essential oil and bioactive constituents. Further, the resulting metabolite diversity could have potential industrial applications in the areas of healthcare, bioremediation, and crop protection.

Multiple Intraspecific Variations of Mitochondrial Genomes in the Green-Tide Forming Alga, Ulva compressa Linnaeus (Ulvophyceae, Chlorophyta)

To gain further insights into intraspecific evolution of Ulva mitochondrial genomes, the mitogenomes of three morphotypes of the green-tide forming alga, Ulva compressa Linnaeus from China and USA, were sequenced and compared with the available data from Ulvophyceae. The U. compressa mitogenomes displayed significantly variable size at intraspecific level ranging from 61,700 to >66,587 bp, due to difference in acquisition of foreign DNA fragments, gain or loss of both group I and II introns, and non-coding intergenic spacer regions. The U. compressa mitogenomes harbored variable gene content ranging from 69 genes (including orfs) in Uco1 to 76 in Uco5, and contained different intron content from 4 introns in Uco3 and Uco4 to 7 in Uco1. A total of 63 genes and only two group IB introns (intron cox1-1107 and cox1-1125) were shared by these five mitogenomes. The U. compressa mitogenomes accumulated much more inverted repeat (IR) elements, ranging from 45 in Uco1 to 88 in Uco2, than that of the other Ulva species (3 - 34). A locally collinear block of eight genes (rps11-rps19-rps4-rpl16-trnR-trnQ-trnE-trnS) with the size of 3,631 bp has been inverted only in Uco1 indicating that the rearrangement event happened after its divergence from Uco2-5 and might be relate to a specific IR element. A majority of the common genes (≈76%) displayed high identity (>98%) among these five mitogenomes, while some low values were observed in six genes mainly due to duplication and insertion/deletion mutations of small DNA fragments. Our study presented the first case of multiple intraspecific variations in ulvophycean mitogenomes, and indicated that the mitogenome will be a valuable tool for understanding the native or nonindigenous nature of the cosmopolitan Ulva species.

The mitochondrial genome of Apion squamigerum (Coleoptera, Curculionoidea, Brentidae) and the phylogenetic implications.

In this article, we present the nearly complete mitochondrial genome (mitogenome) of the weevil beetle Apion squamigerum (Curculionoidea, Brentidae), assembled using data from Illumina next generation sequencing (NGS). This mitogenome was found to be very large, with the total length of 18,562 bp. Two trnM genes were identified. A large non-coding intergenic spacer spanning 1,949 bp occurred between trnI and trnM2. Combined with 111 existing weevil mitogenomes, we conducted phylogenetic reconstructions based on various datasets under maximum likelihood and Bayesian inference. Firstly, phylogenetic analyses robustly supported a sister group of A. squamigerum and Rhopalapion longirostre, namely, that two species of Apioninae (Brentidae) formed a clade. Within the entire Curculionoidea, the Nemonychidae diverged firstly, following the families Anthribidae and Attelabidae. In addition, a large clade comprising the sister families Brentidae and Curculionidae was strongly supported in all trees.

Гибридогенная активность Solidago L. в Северо-Восточной Европе

Two invasive species of goldenrod of North American origin, Solidago canadensis and S. gigantea, widely spread in Europe, are known to be able to form hybrids with the native goldenrod S. virgaurea. In the Kaliningrad Region of Russia and in Lithuania, we found plants morphologically classified as the hybrids S. × snarskisii (= S. gigantea × S. virgaurea) and S. × niederederi (= S. canadensis × S. virgaurea). Analysis of the nuclear ribosomal internal transcribed spacer 1–2 (ITS 1–2) showed the ambiguity of the origin of the alleged hybrids. Thus, in S. × niederederi, the nucleotide substitutions that differentiate the parent species S. virgaurea and S. canadensis had ambiguous readings, indicating heterozygosity, which confirmed the hybridogenic origin of the individuals. However, for many specimens of S. × snarskisii, the hybridogenic origin was not confirmed. Occasionally, hybrids do occur, but appear to be soon absorbed by the maternal species. An analysis of the chloroplast non-coding intergenic spacer rpl32–trnL showed that in these rare cases, the maternal species is S. virgaurea, and the paternal one is S. gigantea.

Genetic Diversity, Phylogeography and Population gene flow of Tunisian Pistaciavera L.

The aim of this paper was to determine how the pistachio trees evolve, and the evolving factors that influence pistachio population establishment. For that, we investigated pistachio genetic structure from some regions of Tunisia, by sequencing two noncoding chloroplastic regions (trnL (UAA) and trnL-trnF intergenic spacer). We found strong genetic diversity among groups, with the absence of high differentiation between population pairs. A deep phylogeographical break separated two major clusters: "El-Guetar and "Gafsa/Sidi-Bouzid. This conclusion is proved by the haplotype networks, the phylogenetic trees, and the molecular variance analysis. Different interpretations were proposed to explain this cytoplasm dimorphism, based on the molecular evolution and demographic history analysis: (1) the domestication events, which are very important to understand the variability between the prospective areas, (2) the gene flow between them, a process that occurs both in time and in space through pollen and seeds, and strongly interacts with the local farming systems, (3) the geographical barriers that exist, which limit gene flow transfer and make particular climatic conditions of the El-Guetar oasis. Given that, the genetic diversity study within Tunisian pistachio cultivars is very useful to contribute to the national management effort for the improvement and conservation of pistachio genetic resources. Moreover, in 2017, Tunisia ranked among the top 10 countries in global pistachio production. Thus, studying the diversity of Tunisian pistachio can make an important impact on global production.

Nuclear and chloroplast genome variability in leek (Allium porrum L.)

The genus Allium L. (Amaryllidaceae), the most numerous among monocotyledonous plants, includes such economically important vegetable crops as onion (A. cepa), garlic (A. sativum) and leek (A. porrum). Leek has a high taste and proven valuable dietary properties and is one of the most popular vegetable crops in Western Europe. Despite a high importance of leek as a vegetable, this species is rarely the subject of molecular genetic studies. The genetic diversity of leeks has never been studied before. Therefore, in this work, we studied the nuclear variability (AFLP) and the chloroplast (nucleotide sequence analysis) genomes using a broad sample. For this work, 65 leek accessions were selected from the collection of the Scientific Center of Vegetable Crops, which included varieties of domestic and foreign breeding. As a result of an AFLP analysis and processing of the DNA spectra obtained, 760 fragments were identified, of which 716 were polymorphic for the leek accessions being analyzed. The calculated genetic distances between the leek samples varied from 0.4 to 0.76, which is comparable to the intraspecific polymorphism of related Allium species (onions, garlic). Analysis of the genomic structure with STRUCTURE 2.3.4 divided the leek samples into seven groups, which generally coincides with the clustering of these samples. To assess the variability of the chloroplast genome, nine sites of the chloroplast genome were sequenced in the leek samples, both non-coding (intergenic spacers rpl32-trnL, ndhJ-trnL, and intron rps16 gene), and protein coding genes (psaA, psaB, psbA, psbB, psbE, petB). The analysis of the sites of the leek chloroplast genome revealed an extremely low level of their polymorphism, only six SNPs were detected in the studied sequences with a total length of about 10,500 bp. Thus, as a result of this work, a high level of polymorphism of the leek nuclear genome was revealed, while the polymorphism of the chloroplast genome was extremely low.

Barcoding assessment of the Citrus species cultivated in the eastern Afghanistan

The establishment of a modern fruit culture in developing countries requests an accurate evaluation of the preexisting germplasm and its health status. This to prevent the possibility to introduce new germplasm which can be easily prey of the endemic diseases carried by asymptomathic host plants. Therefore, after the identification of cases of citrus plants affected by Tristeza virus, a survey of the germplasm cultivated in the Nangarhar valley and some nearby regions was run. The survey was focussed on the identification of the main Citrus species widely cultivated using barcoding analysis of conserved sequences located in the plastid DNA. The sequences of matK and rbcl genes did not show any discriminatory ability while the analysis of the the non-coding psbA-trnH intergenic spacer (psbA-trnH) showed a robust single nucleotide polymorphism (sNP) discriminating C. aurantium from C. sinensis in all analysed samples. These non-coding regions have no known function; thus, much of the variation may result from the spread of mutations unconstrained by selection. Because nucleotide variation in the psbA-trnH spacer region is high, mutational hot spots may be useful to detect species-level variations. According to our study, the afghan citrus germplasm belong to the C. aurantium species which is commonly used in citrus culture as a rootstock. In Afghanistan it is widely cultivated for fresh consumption, without topworking with selected varieties and this may be the reason why symptoms are often mild and cultivation can be anyhow carried on.

Complete mitogenome of Psorodonotus venosus (Orthoptera, Tettigoniidae;Tettigoniinae): Short intergenic spacers shorten the total genome.

Mitogenomes are popular sources of data in evolutionary studies. By development of next generation sequencing the number of total mitogenome in data bases rapidly increased. However, there is still a limited number of total mitogenome known from species of Tettigoniinae. This paper aims to describe the total mitogenome of Psorodonotus venosus (Orthoptera, Tettigoniidae; Tettigoniinae) obtained by NGS reads. The total mitogenome is 15836-15845 bp and consists of 13 protein coding genes (PCG), 22 tRNA genes, two rRNA genes and an AT rich control region as in other metazozans. The mitogenome is AT skewed with 69.5% AT percentage. The genes are ordered as in pancrustacean. Total length of PCGs is 11229 bp, the start codon for all fits ATN pattern and stop codons are incomplete T-- / TA- and rarely complete TAA. Total length of 22 tRNA genes is 1447 bp and their anticodons are identical to other members of Tettigonioidea. The mitogenome contains 12 overlapping regions constituting 41 bp in total. Of these 12 overlapping regions those between trnW-trnC, atp6-atp8, nad4-nad4L, nad6 -cytb and atp6-cox3 gene pairs seem to be conserved. The total length of seven noncoding intergenic spacers is 46 bp. We concluded that P. venosus is one of the species with short mitogenome amongst Tettigonioidea because of limited number and length of noncoding intergenic spacers.

Unusual AT-skew of Sinorhodeus microlepis mitogenome provides new insights into mitogenome features and phylogenetic implications of bitterling fishes

Sinorhodeus microlepis (S. microlepis) is recently described as a new species and represents a new genus Sinorhodeu of the subfamily Acheilognathinae. In this study, we first sequenced the complete mitogenome of S. microlepis and compared with the other 29 bitterling mitogenomes. The S. microlepis mitogenome is 16,591 bp in length and contains 37 genes. Gene distribution pattern is identical among 30 bitterling mitogenomes. A significant linear correlation between A+T% and AT-skew were found among 29 bitterling mitogenomes, except S. microlepis shows unusual AT-skew with slightly negative in tRNAs and PCGs. Bitterling mitogenomes exhibit highly conserved usage bias of start codon, relative synonymous codons and amino acids, overlaps and non-coding intergenic spacers. Phylogenetic trees constructed by 13 PCGs strongly support the polyphyly of the genus Acheilognathus and the paraphyly of Rhodeus and Tanakia. Together with the unusual characters of S. microlepis mitogenomes and phylogenetic trees, S. microlepis should be a sister species to the genus Rhodeu that might diverge about 13.69 Ma (95% HPD: 12.96-14.48 Ma).

Relationship Among Intron Length, Gene Expression, and Nucleotide Diversity in the Pacific Oyster Crassostrea gigas.

Crassostrea gigas is a model mollusk, but its genetic features have not been studied comprehensively. In this study, we used whole-genome resequencing data to identify and characterize nucleotide diversity and population recombination rate in a diverse collection of 21 C. gigas samples. Our analyses revealed that C. gigas harbors both extremely high genetic diversity and recombination rates across the whole genome as compared with those of the other taxa. The noncoding regions, introns, intergenic spacers, and untranslated regions (UTRs) showed a lower level diversity than the synonymous sites. The larger introns tended to have lower diversity. Moreover, we found a negative association of the non-synonymous diversity with gene expression, which suggested that purifying selection played an important role in shaping genetic diversity. The nucleotide diversity at the 100- and 50-kb levels was positively correlated with population recombination rates, which was expected if the diversity was shaped by purifying selection or hitchhiking of advantageous mutants. Our work gives a general picture of the oyster's polymorphism pattern and its association with recombination rates.

Concerted evolution rapidly eliminates sequence variation in rDNA coding regions but not in intergenic spacers in Nicotiana tabacum allotetraploid

Nicotiana tabacum (tobacco) is a natural allotetraploid that formed from two diploid progenitors (N. sylvestris—S-genome, N. tomentosiformis—T-genome) within past 0.2 million years. Previous classical studies have shown that its 35S rDNA has been largely homogenised towards T-genome-like homeologs. However, the degree of conversion at single nucleotide resolution remains unknown. Here, we analysed intragenomic variation of rDNA at high resolution in natural tobacco, synthetic tobacco and the progenitors employing genomic, molecular and cytogenetic methods. In synthetic tobacco, we identified 13 highly (≥10% units) polymorphic sites in the 18S-5.8S-26S coding region. In contrast, only a single polymorphic site was detected in natural tobacco, indicating that gene conversion has removed most of the polymorphisms over shallow evolutionary times. However, the non-coding 26S-18S intergenic spacer (IGS) was highly polymorphic in both natural (57 polymorphic sites) and synthetic tobacco (128 polymorphic sites). In natural tobacco, most (64%) IGS polymorphisms were inherited from the N. tomentosiformis progenitor, while 36% appeared de novo indicating rapid rates of sequence divergence of IGS. FISH revealed that the T-genome-like units (harbouring N. tomentosiformis-type IGS) occurred on all four loci in tobacco variety 095-55, including those loci derived from N. sylvestris progenitor, while the variety SR-1 retained 1–2 S-genome loci unconverted and transcriptionally silenced. We discuss potential caveats associated with experimental and in silico approaches used for determination of rDNA polymorphisms. We also hypothesise that polyploidy-associated gene conversion may eliminate mutated and non-functional genes that have accumulated in progenitor genomes, thereby contributing to success of polyploidy species.